An apparatus for making a plastic foil by blowing an extruded tube into an expanded thermoplastic tube which can be flattened, generally comprises a rotating extruder head with at least one circular outlet for the extruded thermoplastic tube, an air blowing device for inflating the tube and a cooling device including two or more circular cooling air outlets surrounding the circular outlet gap of the extruder head. Cooling air duct structures are connected to air blowers. The cooling device can be held fixed or can rotate in the same direction as or counter to the extruder head.
In the known apparatus an aerodynamically central cooling air feed device is provided which supplies all the circular cooling air outlets.
The cooling air flows issuing from the individual cooling air outlets are not controllable independently of each other and of course not when the cooling air outlets have a single control device. This impairs the homogeneous cooling of the foil tube over its circumference.
To eliminate this disadvantage a separate blower could be provided at each cooling outlet with a separate system of feed ducts from the blower to its cooling air outlet. This however is unnecessarily expensive.